Shaped glass sheet and a process for the preparation thereof

ABSTRACT

A shaped glass sheet having a sharp bend is provided, wherein a radius of curvature of an outer circumference of the bend transverse to the bend is at most 4 times a thickness of the glass sheet and the straight sections of the shaped glass sheet are substantially flat. A process for the preparation of the shaped glass sheet comprises applying an electrically conductive material on a surface of a glass sheet to form a bend line, generating a graded temperature profile transverse the bend line, and then applying an electrical potential to the bend line. In another process, a width or thickness of a bend line is not uniform or a bend line is discontinuous in the longitudinal direction of the bend line, no particular temperature profile is required and, upon applying an electrical potential, arc discharge occurs along the bend line to melt the glass rapidly.

FIELD OF THE INVENTION

The present invention relates to a shaped glass sheet, particularly, foruse in buildings and general industries, more specifically a bent glasssheet composed of more than one plane parts with no seem line at a bendposition, and a process for the preparation thereof. The invention alsorelates to a glass sheet structure composed of at least two bent glasssheets with no seem line at a bend position, where the glass sheets arepositioned with a space to each other.

BACKGROUND OF THE INVENTION

Glass sheets are used in the exterior and interior of buildings and ingeneral industries, for instance, in windows, show windows, wallmaterials, doors, partitions, display cases, water tanks and furniture.At a corner of a window, show window or display case, two glass platesare joint at a desired angle with a frame made of metal, wood orplastic, and the joining parts between the glass plates and the frameare bound with a calking material such as silicone rubber and a sealingmaterial. However, appearance and seeing through are disturbed by theframe at a corner. In addition, a stainless steel frame increases costs.

Alternatively, an edge of a glass sheet is matched with an edge ofanother glass sheet without using a frame and the matched edges areadhered with a calking material such as silicone rubber and a sealingmaterial. However, appearance and seeing through at a corner is not soimproved even in such a structure. Further, the adhered corner is weakin strength. In addition, calking materials and sealing materials havelimited weatherability. Therefore, the life of the adhered part isrelatively short in general.

In order to remove the aforesaid drawbacks, a method is known where aglass sheet is heated and bent to have a bend of a desired angle. Forinstance, a usual float method glass sheet is placed horizontally on aframe which has a desired angle, the whole glass sheet is heated atabout 500° to 580° C. and a position of the glass sheet to be bent isfurther heated locally to about 700° to 750° C., and then the glasssheet is molded on the frame by gravity or other external force to havea bend of a desired angle. However, a radius of curvature of the bend isrelatively large. When a glass sheet is 4 mm thick for instance, aradius of curvature is at least 35 to 40 mm. Accordingly, a window,water tank, show window or display case with such a bend will largelydistort an object. Further, because a relatively broad area near thebend of a glass sheet is disposed to a temperature above an annealingpoint, warpage is seen after bending in such a relatively broad area.Thus, straight parts have large distortion. FIG. 10 is an illustrativecross-sectional view of a bend area of a glass sheet which is formed inthe aforesaid method. A radius of curvature at a bend, B₁, isapproximately 10 times a thickness of the glass sheet, d. Parts, AB₁ andAB₂, are transitional areas with a length several times the thickness,d, between straight parts, A₁ and A₂ and a bend, B₁, and are distortedwith bad optical properties. In addition, straight parts, A₁ and A₂,cannot maintain their original flatness as before the heat processingdue to exposure to a high temperature, and have seeing throughdistortion and reflection distortion.

U.S. Pat. No. 3,762,903 discloses a process for bending a glass sheet toa relatively sharp angle, which comprises applying a layer ofelectrically conducting material to at least one surface of the sheetalong a line about which it is desired to bend the sheet, applying anelectrical potential across said line of a sufficient magnitude and fora time adequate to heat the sheet in the area immediately adjacent saidline to a temperature above the bending point of the glass, and bendingsaid sheet along said line to form said relatively sharp angle therein.More specifically, a glass sheet is placed on a bending mold such as aV-shaped frame. The glass sheet and the mold are heated to a temperatureof 900° to 1,150° C. F(i.e., 482° to 621° C.) in a furnace. Upon theglass sheet reaching the desired overall temperature, an electricalpotential is applied to the line of the electrically conducting materialto heat the glass sheet immediately adjacent the line to a temperatureabove the bending temperature of the glass, for instance, above 1,200°F., i.e., 649° C. Then the glass sheet is bent by gravity intoconformity with the bending mold. Typically, a potential of 45 voltsmaximum is applied in a current of 3 amperes for 15 minutes.

Windshields and backlights for automobiles are intended there and,therefore, the illustrative thickness of a glass sheet is rather small,i.e., 0.090 inch or 2.3 mm.

U.S. Pat. No. 3,762,904 discloses a process which is similar to theaforesaid process of U.S. Pat. No. 3,762,903, but is different in that agroove is formed in a surface of a glass sheet along a line about whichit is desired to bend the glass sheet and an electrically conductingpath is formed on an ungrooved surface of the glass sheet opposite thegroove.

It is known to fit plural glass sheets in a window in dual or morestructure for protection against cold, particularly, in cold districts.Such a dual or more structure of glass sheets is often used also in showwindows, display cases and the like as an exterior for the same purposeas stated above. A corner part of such windows, show windows and displaycases is constructed in such a way that two or more glass sheets arefaced to one another in a desired space with spacers being inserted, andthe spacer areas are sealed with a calking agent such as silicone rubberand a sealing agent. Then, a set of glass sheets thus sealed is boundwith another set of glass sheets at a desired angle with a frame made ofmetal, wood or plastic. In this prior art construction, the sealingagent such as butyl rubber for sealing the gap between glass sheet edgeshas insufficient weatherability. Therefore, a care is needed to protectthe sealed part from direct irradiation with lights. To this end, theframe has a sufficiently large depth, which however obstructs appearanceand seeing through at a corner. Further, a stainless steel frameincreases costs. To solve the aforesaid shortcomings, it may be thoughtto match an edge of a glass sheet with an edge of another glass sheetwith no frame and to adhere the matched edges to each other with acalking material such as silicone rubber and a sealing agent. However,the sealing agent such as butyl rubber is directly irradiated withlights and then its life is too short for practical use.

SUMMARY OF THE INVENTION

The present invention provides a process for the preparation of a shapedglass sheet with a very sharp bend and substantially flat straightsections extending from the bend.

That is, the present invention is a process for the preparation of ashaped glass sheet having at least one linear bend and two straightsections extending from the bend, comprising

applying an electrically conductive material to at least one surface ofsaid glass sheet to form a linear bend line along a line about which itis desired to bend the glass sheet,

heating areas of the glass sheet spaced at least 3 cm from the bend linetransverse to the bend line to temperatures between 200° C. and 500° C.lower than a softening point of the glass sheet,

heating areas of the glass sheet spaced less than 3 cm from the bendline transverse to the bend line to temperatures higher than saidtemperatures of the areas at least 3 cm from the bend line, but at least50° C. lower than the softening point of the glass sheet,

applying an electrical potential to the electrically conductive materialof the bend line to heat the glass along the bend line to at least thesoftening point of the glass sheet and then

bending the glass sheet along the bend line to form a shaped glass sheethaving a sharp bend, wherein a radius of curvature of an outercircumference of the bend transverse to the bend is at most 4 times athickness of the glass sheet and the straight sections of the shapedglass sheet are substantially flat.

In another process of the invention, a very narrow area of a glass sheetis heated to at least a softening point of glass in a very short periodof time by a special means. Accordingly, two-dimensional stress causedin the glass sheet during heating and bending is so small that noparticular care is needed for a temperature profile in the glass sheet.

That is, the present invention also provides a process for thepreparation of a shaped glass sheet having at least one linear bend andtwo straight sections extending from the bend, comprising

applying an electrically conductive material to at least one surface ofthe glass sheet to form a linear bend line along a line about which itis desired to bend the glass sheet, a width or thickness of said bendline being not uniform or the bend line being discontinuous in thelongitudinal direction of the bend line,

applying an electrical potential to the electrically conductive materialof the bend line to cause arc discharge along the bend line to therebyheat the glass along the bend line to at least a softening temperatureof the glass sheet and then

bending the glass sheet along the bend line.

The present invention relates also to a shaped glass sheet thusproduced. That is, the invention provides a shaped glass sheet having atleast one linear bend and two straight sections extending from the bend,characterized in that a radius of curvature of an outer circumference ofthe bend transverse to the bend is at most 4 times a thickness of theglass sheet and the straight sections of the shaped glass sheet aresubstantially flat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a glass sheet temperature profile in accordancewith the process of the invention.

FIG. 2 is another example of a glass sheet temperature profile inaccordance with the process of the invention.

FIG. 3 shows illustrative bend line patterns which are used to cause arcdischarge along a bend line according to the process of the invention.

FIG. 4 shows enlarged cross-sectional views of parts of glass sheetsnear grooves in accordance with the invention.

FIG. 5 is a conceptual cross-sectional view of a bend part of a shapedglass sheet according to the invention.

FIG. 6 is schematic views of examples of the shaped glass sheetaccording to the invention.

FIG. 7 is a schematic view of a glass sheet structure of the invention.

FIG. 8 is a graph of an electrical power vs. time which was adoptedaccording to the invention in Example 2.

FIG. 9 is a glass sheet temperature profile transverse to a bend linewhen glass along the bend line was heated according to the invention inExample 2.

FIG. 10 is a conceptual cross-sectional view of a bend part of a bentglass sheet produced in the prior art.

FIG. 11 is a cross-sectional view of a glass sheet structure which isunified with a frame.

PREFERRED EMBODIMENTS OF THE INVENTION

In the process of the invention, a glass sheet is heated in a particulartemperature profile which reduces two-dimensional internal stress causedby local heating with an electrical current, and prevents the glasssheet from cracking.

Of course, the internal stress can be reduced by overall heating of aglass sheet in a furnace at a rather high temperature, as described inU.S. Pat. No. 3,762,903. Then, it is hard to attain a very sharp bend asdefined above. In addition, transitional areas take place between a bendand straight sections, where optical distortion is apparent. Further,flatness of the straight sections is damaged by such overall heating ata rather high temperature and by gravity which causes bending of theglass sheet. In this U.S. Patent, a rather thin glass sheet ismentioned. Even so, the aforesaid disadvantages are seen. Furthermore,the whole glass sheet and a frame supporting the glass sheet are heatedtogether, which requires a large scale furnace and a lot of energy.

In the process of the invention, glass along the bend line (i.e., glassunder and immediately adjacent the bend line) is heated to at least thesoftening point of the glass, preferably in a range of from thesoftening point to 140° C. plus the softening point. If the temperatureexceeds this preferred upper limit, distortion will appear in straightsections near the bend of a bent glass sheet, and the heat economy ofthe process is also worse.

In the present invention, a softening point of a glass sheet means atemperature at which the viscosity of the glass is about 10⁸ poise,where the glass changes its shape due to its own weight. The softeningpoint of a usual float method glass is about 740° C.

In a preferred embodiment of the process according to the invention,areas of a glass sheet spaced at most 5 cm from edges of the glass sheetfacing the bend line are heated to temperatures (T₁) 400° C. to 500° C.lower than a softening point of the glass sheet, areas of the glasssheet at least 3 cm from the bend line transverse to the bend line andat least 5 cm from the edges of the glass sheet facing the bend line areheated to temperatures (T₂) higher than said temperatures T₁, and theareas spaced less than 3 cm from the bend line are heated totemperatures higher than T₂, but at least 100° C. lower than thesoftening point of the glass sheet, before the glass sheet is bent.

To attain the particular temperature profile according to the invention,the whole glass sheet is preferably heated by radiation and/or heattransfer, glass in areas near the bend line is subsequently heated byradiation, and then glass in an area immediately adjacent the bend lineis electrically heated. The heat transfer heating of the whole glasssheet is preferably effected by putting glass in areas other than thearea near the bend line between heating plates. For instance, flexibleheaters for heat transfer are brought into contact with the bothsurfaces of the glass, on which a heat insulating material is put andpressed so as to attain close contact between the flexible heaters andthe glass surfaces. The radiation heating of the glass in areas near thebend line is preferably conducted with a seeds heater or a halogen lamp.For instance, a rod shape seeds heater with a diameter of 15 mm isplaced approximately 40 cm above and in parallel with the bend line.

Any known electrically conductive materials may be used to form the bendline. Preferred examples for them include conductive paints containingcarbons such as graphite, metals such as silver, copper, nickel,chromium, palladium and platinum, oxides such as tin oxide, or siliconcarbide; and metals or alloys such as iron-nickel-chromium alloy,copper-tin alloy and aluminium alloys. More specifically, for carbontype conductive paints, Bany Hyte F-525W-1, Nippon Kokuen Shoji Co., maybe named. H 9100, Hokuriku Paint Co., and D-1230 (Kai), FuJikura KaseiCo., may be named for silver type conductive paints. Among these, anyproper one may be selected depending upon the size and thickness of aglass sheet, electrical voltage used, electrical resistance of a bendline applied, adhesiveness to glass and coloring of glass due todiffusion of metallic ions. Carbon type ones are convenient, as controlof electric power is easy to do and they may be completely removed incombustion by blowing oxygen after the heating step, e.g., in thebending step.

Pasty conductive paints may be applied on the surface of a glass sheetin a screen printing, anastatic printing, spraying or transferringprocess. Metals or alloys may be applied in a metal flame coating, CVDor pyrolytic process. In a metal flame coating method, a mask having aslit corresponding to a bend line is placed on a glass sheet. A metal asmentioned as a conductive material is melt by electrical arc or flame,which is blown through a nozzle by compressed air to coat the glasssurface in a pattern of a bend line. In any one of the aforesaidmethods, a conductive material may be applied precisely at a desiredposition and the applied bend line adheres closely to the surface of aglass sheet. However, the screen printing and the metal flame coatingprocess are preferred for accuracy and convenience. The number of bendlines to be applied is same as the number of bends desired in a shapedglass sheet product, and is at least one in a glass sheet. A pluralityof bend lines may be formed in parallel with one another or not inparallel.

The width of a bend line may vary depending upon the aforesaid factors,i.e., the size and thickness of a glass sheet, electrical voltage used,electrical resistance of a bend line, adhesiveness to glass and coloringof glass due to diffusion of metallic ions, but is preferably 0.3 to 3times, preferably 0.5 to 1.5 times, the thickness of a glass sheet in astraight section. If the width of the bend line is more than 3 times thethickness of a glass sheet in a straight section, a too wide range ofthe glass sheet is heated around its softening point, so that a bendobtained will not be sharp enough. Meanwhile, if the width is less thanthree-tenths of the thickness of a glass sheet, sufficient heating isnot attained and the thickness of a bend obtained will be too small. Thethickness of a bend line layer may vary depending upon a type of theelectrically conductive material and a method of applying the bend line,and should be such that glass is constantly heated to a predeterminedtemperature in a predetermined period of time. The thickness ispreferably one-tenth to several hundreds μm, and particularly 10 to 100μm in screen printing and 30 to 300 μm in flame coating.

A bend line may be applied on a surface of a glass sheet, both surfaces,one surface and two end faces, or in a loop shape connecting bothsurfaces and two end faces. For a rather thin glass sheet, a bend lineis usually applied only on the surface which will become an outercircumference of a bend. For a thick glass sheet, it is preferred toapply a bend line on both surfaces. When dielectric heating is adoptedas will be described below, a bend line in a continuous loop shape isrequired.

The glass sheet to be bent in the invention may be those used inbuildings or general industries. However, it is characteristic of theprocess of the invention that a thick glass sheet with a thickness of 3mm or more can be bent with a very sharp bend and flat straightsections. The glass sheet to be bent may be produced in any knownmethods. However, preferred are float method glass sheets which areformed on molten tin. The thickness, shape and size of the glass sheetare not limited and may be decided for particular purposes. Compositionof the glass is not limited, either. Glass sheets having differentsoftening points may be used, such as soda lime glass, borosilicateglass and high strength crystallized glass. Various types of glasssheets may be used, such as general purpose glass sheets, polished glasssheets and cut glass sheets. Glass sheets may be surface-treated, forinstance, by thermic rays reflection coating, non-reflection coating,printing of a particular pattern or surface etching.

An electrical potential is applied to a bend line, preferably, in any ofthe following manners. Solid terminals of a proper shape are broughtinto contact with a bend line at edges of a glass sheet, to which acurrent is applied, or electrically conductive high temperature gasflame generating lithium ions or the like is brought into contact with abend line at edges of a glass sheet, through which flame a current isapplied. Alternatively, a bend line is formed in a loop shape, to whicha high frequency electric source is connected for dielectric heating.The electric source used in the aforesaid manners may be industrialalternating electric source of 50 or 60 Hz, high frequency electricsource of 100 KHz to 50 MHz or direct current electric sources. Any ofthe aforesaid types of electric sources may be used in the manner ofusing solid terminals. High frequency electric sources are preferred forthe manner of using conductive high temperature gas flame. Dielectricheating allows only the use of high frequency electric sources. Besidesthe manner of applying a current, the size of a glass sheet is alsotaken into consideration in selection of the type of an electric sourceused.

Control of the aforesaid electric heating is conducted withconsideration of a potential, a current, period of time of turning onelectricity, a thickness and size of a glass sheet, a width of a bendline, a temperature profile in the whole glass sheet, a speed of bendinga glass sheet and a final angle of a bend. Preferably, a current andperiod of time of turning on electricity are controlled. Manycontrolling factors are involved as mentioned above and, further, atemperature profile in the whole glass sheet changes with time.Accordingly, it is preferred to control the temperature by a computer.In general, turning on electricity for 30 seconds to 5 minutes,particularly 1 to 3 minutes, allows sufficient heating, i.e., tens tohundreds watts per 10 cm of a bend line.

FIG. 1 shows an example of a temperature profile in a glass sheet whichis attained according to the invention. Glass sheet, G, is 800 mm long,400 mm wide and 4 mm thick. Its softening point is 740° C. A bend line,L, is drawn at the center of the glass sheet in the longitudinaldirection. In the lower figure, a temperature profile along line XX'shown in the upper figure is depicted, where the glass is heatedaccording to the invention. Point, p, in glass sheet G is marked also inthe temperature profile. In the temperature profile, the area of about 6mm or less from the bend ine is about 800° C. In the areas outside theaforesaid area, which areas were heated by radiation, temperaturesdecrease abruptly in a range of 25 mm from bend line L. At point p,which is 100 mm far from bend line L, the temperature is about 450° C.,which temperature prevails almost constantly up to the edges. Thus, thetemperature profile is made very sharp near bend line L in order tomaintain the flatness in areas adjacent the bend line in the invention.Nevertheless, cracking of a glass sheet can be prevented by controllingthe temperature profile of the whole glass sheet properly as above.

In addition to the temperature profile transverse to the bend line, itis preferred to attain a predetermined temperature profile also in thedirection parallel to the bend line. That is, in each temperatureprofile in each line parallel to the bend line and transversely spaced10 cm from the bend line, the temperature of an area near the edge ismade higher than the temperature of a central position by 30° to 150°C., more preferably by 50 to 100° C. By this temperature profile,breakage of a glass sheet due to two-dimensional internal stress is moresuitably prevented. Such a predetermined temperature profile parallel tothe bend line may be attained by radiation heating and/or heat transferheating for heating areas near the edges.

FIG. 2 shows an example of temperature profiles in the case where atemperature curve in a direction parallel to a bend line is made uneven.Glass sheet, G₁, is the same as that in FIG. 1. A temperature profilealong line X₁ X₁ ' and a temperature profile along line Y₁ Y₁ ' for aglass sheet heated according to the invention are illustrated in FIG. 2.Position, P₁, in glass sheet G₁ is shown also in the two temperatureprofiles. In the temperature profile along line X₁ X₁ ', areas spacedabout 6 mm from bend line L₁ on the right and left sides are atapproximately 800° C., and temperatures decrease relatively moderatelywith a location moving toward the edges. At point P₁, which is about 100mm far from bend line L₁, the temperature is approximately 350° C.Temperatures decrease more moderately with a location moving toward theedges, and are finally about 250° C. at the edges. Meanwhile, in thetemperature profile along line Y₁ Y₁ ', the temperatures in areas nearthe edges are made higher than the temperature at a central position,which is important for preventing a glass sheet from cracking due totwo-dimensional stress. That is, although the temperature profile alongline X₁ X₁ ' is made very steep near bend line L₁ to maintain theflatness of the glass sheet near the bend line according to theinvention, cracking of the glass sheet is preferably prevented byproperly controlling temperature profiles in the whole glass sheet. Lessenergy is required to heat a glass sheet in the embodiment of FIG. 2than in FIG. 1.

The heating is usually conducted in air, but this is not restrictive.

To bend a heated glass sheet along a bend line at a predetermined angleaccording to the invention, a glass sheet-holding mechanism is used. Abending speed, a bending force and a final bend angle are preciselycontrolled by the glass sheet-holding mechanism, so that generation ofcracks at the bend area is prevented and a predetermined shape of thecross section and a predetermined thickness at the bend are attained.Preferably, the glass sheet-holding mechanism is provided with aradiation heating means and/or a heat transfer heating means which areused to heat straight sections of a glass sheet. For instance, flexibleheaters for heat transfer heating are brought into contact with bothsurfaces of a glass sheet, on which a heat-insulating material isplaced. The glass sheet-holding mechanism has plates attached to arms,by which the glass sheet is clamped via the insulating material. Thearms are moved by power to bend the glass sheet at a predeterminedangle. An internal angle of two adjacent straight sections is preferablymade 60 to 160 degrees. The bending operations can be completed in avery short period of time, preferably in 1 to 5 minutes in general.

The electrically conductive material of the bend line is usually removedafter the heating and shaping steps. Most of the components of theelectrically conductive material burn or scatter gradually during theheating, and almost no conductive material remains after the shaping insome cases. From this point of view, carbon type conductive paints arepreferred. Some conductive materials do not completely burn todisappear, but remain on the glass surface, depending upon types of theconductive materials and thickness of their layer. In such a case, theremaining material can be removed by blowing oxygen after the heatingand shaping steps to completely burn it, or by mechanical or chemicaltreatment to the glass cooled, such as polishing. When a metal typeconductive material is used, the metallic component, usually, diffusesinto the glass. When it is desired to leave a line from a conductivematerial in a completed shaped glass sheet for a design purpose, a bendline may be formed with a conductive material having a particularcomposition which will offer a desired color tone.

The shaped glass may be after-treated as usual. For instance, it may becooled slowly or rapidly to obtain a slow-cooled glass or tempered orhighly tempered glass. It may be subjected to surface treatment such asthermic rays reflection coating, non-reflection coating, printing of aparticular pattern, and surface etching.

An improvement has now been found, which makes it possible to heat glassalong a bend line to at least a softening point of the glass in a muchshorter period of time and, also, to minimize two-dimensional internalstress generating in a whole glass sheet during heating of a bend linezone.

This method comprises applying an electrically conductive material to atleast one surface of the glass sheet to form a linear bend line along aline about which it is desired to bend the glass sheet, a width orthickness of said bend line being not uniform or the bend line beingdiscontinuous in the longitudinal direction of the bend line, applyingan electrically potential to the electrically conductive material of thebend line to cause arc discharge along the bend line to thereby heat theglass along the bend line to at least a softening temperature of theglass sheet and then bending the glass sheet along the bend line.

In a bend line of the aforesaid form, electrical resistance is larger inpoints where the width or thickness of the bend line is smaller or wherethe bend line is discontinued (hereinafter referred to generically as"narrower points"). Accordingly, when a high electrical potential isapplied to the bend line, arc discharge takes place continuously,stepping over the narrower points. Then, the temperature at these pointsbecomes high enough to melt the glass, resulting in formation of meltspots. The electrically conductive material burns, volatilizes ordiffuses into glass at a high temperature. Accordingly, the amount ofthe electrically conductive material decreases in places where a broaderor thicker part of the bend line adjoins the melt spots, as a result ofwhich arc discharge occurs also there, and finally melt spots areconnected with one another to form a melt line. In the melt line, thetemperature is 700° C. or higher, and sodium ions generate so that theglass has better electrical conductivity and electricity moves on thesurface part of the glass. With the further lapse of time, theconductive material in the bend line disappears further and arcdischarge on the glass surface comes to cease. In addition, thetemperature increase in the glass surface reaches the top due to heatdissipation. As the results, the temperature in points immediately belowthe melt line becomes higher than that on the glass surface, and theelectricity then moves inside the glass immediately below the melt linewhere the temperature is higher and electrical conductivity is higher.Accordingly, the inside of the glass immediately below the melt line isheated to melt by Joule heating. With repetition of the aforesaidphenomena, electricity will move more inside the glass to effect heatingthere. As the results, it is believed that the electrical currentconcentrates in a line central in the direction of the thickness of theglass and perpendicularly below the bend line and the glass has atemperature profile having a peak at the central position in thedirection of thickness. According to visual observation, glass meltsvery rapidly, starting at the surface and reaching the center. Settingaside detailed theory, the glass is melt very locally along the bendline through the whole thickness of the glass sheet. This is completedin a very short period of time, i.e., several seconds to 30 seconds.With a glass sheet of 5 mm thick, a period of 5 seconds is enough, andthe glass in the area of a bend line even hangs down by its own weightin 10 seconds.

Therefore, heat transfer toward straight sections of a glass sheet isless, and the temperature in areas near a bend line do not reach thesoftening point. Bending of a glass sheet thus heated looks like weldingof two glass sheets. Accordingly, it is possible to produce a shapedglass sheet with a very sharp bend and very flat straight sections whichextend up to near the bend.

FIG. 3 shows examples of patterns of bend lines used for arc dischargein the process of the invention. The unit of the size indicated in thedrawing is millimeter. A width of a bend line at its widest point doesnot depend much upon a thickness of a glass sheet, and is preferably 0.5to 5 mm, particularly 1 to 3 mm. If the largest width is less than 0.5mm, heating with sufficient electric energy cannot be effected.Meanwhile, if it exceeds 5 mm, a too wide range of a glass sheet isheated around its softening point, so that a bend will not be sharpsufficiently. The narrowest width in each bend line pattern ispreferably 1 mm or less, particularly 0.5 mm or less. If the narrowestwidth exceeds 1 mm, arc discharge is difficult to occur. A bend line maybe discontinuous, preferably, with gaps of 0.1 to 2 mm, particularly 0.5to 1.0 mm. If the gap exceeds 2 mm, arc discharge is difficult to occur.

A bend line is composed of repeated patterns which are defined by thewidest points and the narrowest points or by the gaps, and each patternis preferably 0.3 to 10 mm long, more preferably 1 to 3 mm long, in thelongitudinal direction of the bend line. If it exceeds 10 mm, a timeperiod required for heating is too long. Meanwhile, if it is less than0.3 mm, it is difficult to precisely form a bend line in a predeterminedshape. The thickness of the bend line may vary depending upon a type ofthe electrically conductive material and a method of applying the bendline, and should be such that glass is constantly heated to apredetermined temperature in a predetermined period of time. Thethickness is preferably one-tenth to several hundreds μm, andparticularly 10 to 100 μm in screen printing and 30 to 300 μm in flamecoating.

A bend line may be such that a thickness of a bend line layer varies inthe longitudinal direction. The largest thickness is preferably 10 to 50μm and the smallest thickness is preferably 5 to 15 μm. A width of sucha bend line is preferably 0.5 to 5 mm. A pattern composed of a thickerpart and a thiner part is preferably 0.3 to 10 mm in length. Forinstance, a bend line may have a width of 1.0 mm, the largest thicknessof 30 μm, the smallest thickness of 10 μm and a length of a pattern of 1mm.

A manner of applying an electrically conductive material to a surface ofa glass sheet to form a bend line, a manner of applying an electricalpotential to the electrically conductive material of the bend line and amanner of bending the glass sheet may be same as described above, butthe electrical potential here must be sufficiently high to cause arcdischarge, preferably several thousands volts to twenty thousands voltswith a current of, preferably, 0.5 to 2 amperes.

Control of the electrical heating is conducted with consideration of athickness and a size of a glass sheet, a largest width and a smallestwidth or a gap size of a bend line, a speed of bending a glass sheet anda final angle of a bend. Preferably, a current and a period of time ofturning on electricity are controlled by a computer. In general, turningon electricity for several seconds to 30 seconds allows sufficientheating, i.e. tens to hundreds watts per 10 cm of a bend line. Thus, theheating time is shortened to from approximately one half toone-twentieth of that needed in the aforesaid embodiment where atemperature profile is controlled. For instance, heating time of 5seconds is enough for a glass sheet of 3 mm thick. This process with arcdischarge is effective particularly for thicker glass sheets.

Heating is effected in a very narrow band of a glass sheet in thisprocess with arc discharge. Therefore, two-dimensional stress caused issmall and lasts for only a short period of time, so that a danger for aglass sheet to break is small. This will be easily understood byconsidering an imaginary case where a glass sheet is melt in a band ofan infenitesimal width and an area nearby is at normal temperature, thenthe glass sheet will never break.

It is preferred in the process of the invention using arc discharge thata whole glass sheet is heated approximately uniformly at a temperaturebelow an annealing point of glass, more preferably 0° to 200° C. lower,particularly 50° to 100° C. lower,than an annealing point in advance ofthe heating of a bend line area. Two-dimensional internal stress causedin a glass sheet in local heating with arc discharge is made very smallwith such a convenient advance uniform heating at a relatively lowtemperature. Thus, breakage of a glass sheet is safely prevented. Anannealing point of glass generally means a temperature at which aviscosity of glass is approximately 10¹³ poise. Any known method may beused for the uniform heating. For instance, a whole glass sheet is putin an electric furnace controlled at a predetermined temperature, whichis easy and practical.

In the process of the invention with arc discharge, it is preferred thata groove is formed in a surface of the glass sheet along a line aboutwhich it is desired to bend the glass sheet, and the glass sheet is bentinwards toward the groove side of the glass sheet to thereby fuse theglass defining the groove together.

A bend line is formed with an electrically conductive material in ornear a groove. It is preferred that the electrically conductive materialto form the bend line is applied in at least area of the followlocations: (1) on surfaces of glass defining the groove, (2) on an areaimmediately adjacent the groove, and (3) on the surface of the glasssheet opposite the groove. A means of forming a groove in a glass sheetis known. For instance, a V cut machine for glass plates supplied fromSuzuki Shokai Ltd. may be used. The number of grooves is same as that ofbends desired in a shaped glass sheet product, and at least one in aglass sheet. When a plurality of grooves are formed, they may beparallel to one another or not parallel. Profile of a groove may betriangular or square, but preferably triangular. A size of the profilemay be properly decided in consideration of a thickness of a glasssheet, an angle of a bend of a shaped glass sheet and so on. In general,a depth of a groove is decided by a desired radius of curvature of thebend. It is preferably one fifth to one half of a thickness of a glasssheet. A bottom angle of a triangular groove is preferably same as anangle of a bend of a shaped glass sheet. To perpendicularly bend a glasssheet with a thickness of 5.0 mm, an illustrative groove has atriangular profile with a right bottom angle and a depth of 2.0 mm. Thegroove is formed generally only on a surface toward which a glass sheetis bent.

FIG. 4 is enlarged cross-sectional views of parts of glass sheets neargrooves and shows where a bend line is applied. Black thick lines in thedrawing show cross sections of bend line layers, but their scale isunreal for convenience of drawing. As shown in FIG. 4, (A) for instance,a bend line is applied on surfaces of glass defining a groove. When anelectrical potential is applied to the bend line, arc discharge occursso that the glass defining the groove is melted very locally and, uponthe glass sheet being bent, fused together. Accordingly, a product has alarge strength at a bend.

In other embodiments, bend lines may be applied on right and leftsurfaces of a glass sheet immediately adjacent a groove as shown in (B),on a glass sheet surface opposite a groove as in (C), or both as in (D).

In further other embodiments, bend lines may be applied on surfaces ofglass defining a groove and on surfaces of a glass sheet immediatelyadjacent the groove as in (E), on surfaces of glass defining a groove,on surfaces of a glass sheet immediately adjacent the groove and on aglass sheet surface opposite the groove as in (F), or on surfaces ofglass defining a groove and a glass sheet surface opposite the groove asin (G). In these embodiment, glass on the surfaces of a groove and glassin a part to be bent are rapidly heated and therefore a radius ofcurvature of a bend can be smaller, which is particularly desired.

A bend line may be applied at a bottom of a groove as in (H), or inanother groove formed on a surface opposite a first groove as in (I). Ineither embodiment, a relatively thick glass sheet may be used, and thesoftened and melted part is very narrow. In the embodiment (I), thegroove for a bend line is formed on the surface which is stretchedduring bending and, upon bending, will be flatened to disappear.

In the above, grooves with a triangular profile are illustrated, butthis not restrictive. In (A) through (I), grooves may have a squareprofile. In (J) which corresponds to (C), the cut-off area in a grooveis larger. This is efficient, particularly, for a thick glass sheet whena radius of curvature of a bend is made less than a thickness of a glasssheet. However, it is advisable to decide a shape and size of a groovein consideration with an angle of a bend so that no interstice remainsat the groove position after bending.

A width of a bend line depends upon a place of a bend line, a shape andsize of a groove, a thickness of a glass sheet, an angle of a bend, anelectrical potential and a shape of a bend line. In (A), (E), (F) or (G)where a bend line is applied on the surface of a groove, a bend line maybe applied on the whole surface of a groove or partially. When a glasssheet is relatively thick, a bend line may be applied in a bottom of agroove, so that a softened and melted part will be very narrow, but,with sufficient heating as in (H). In (B) where a bend line is appliedin areas immediately adjacent a groove, or (C) and (J) where a bend lineis applied on the surface of a glass sheet opposite a groove, the widthof a bend line is preferably 0.2 to 3 times a thickness of a glass sheetat a position of the groove, particularly one half to equal to that. In(D), (F) and (G) where more than one bend lines are applied, even if thewidth of each bend line is smaller than that mentioned above, sufficientheating can be effected.

The shaped glass sheet produced in the process of the invention has avery sharp bend and very flat straight sections. This is particularlytrue in the process using arc discharge heating.

That is, the present invention also provides a shaped glass sheet havingat least one linear bend and two straight sections extending from thebend, characterized in that a radius of curvature of an outercircumference of the bend transverse to the bend is at most 4 times,preferably at most twice, a thickness of the glass sheet and thestraight sections of the shaped glass sheet are substantially flat. Ifthe radius of curvature is more than 4 times a thickness of a glasssheet in straight section, a bend distorts a substance which is seenthrough there. In an extreme case, a substance is enlarged by a bend.There may be a transitional area between a bend and a straight section,in which area the glass sheet is slightly warped to loose flatness. Awidth of the transitional area is preferably at most twice a thicknessof a glass sheet, particularly not more than that. If the width exceedstwice the thickness, a straight section near a bend distorts a substancewhich is seen through there. In the bent glass sheet of the invention,the straight sections are substantially flat. In the case of floatmethod glass sheets for instance, its original flatness before thepresent process being applied is maintained as such.

FIG. 5 is a conceptual cross-sectional view of a bend part of a shapedglass sheet of the invention. R indicated in the drawing is the radiusof curvature defined in the invention. Points P and Q are points where acurved surface starts. Outsides points P and Q in a shaped glass sheetaccording to the invention there may be transitional area S which isseemingly flat, but warps slightly to loose flatness and has relativelybad optical properties. In a glass sheet having a bend in the prior art,a radius of curvature is remarkably large as shown in FIG. 10, a boarderbetween a flat part and a bend is unclear and there is a longtransitional area between a flat part and a bend, which warps slightlyto loose flatness. This is apparently different from the glass sheet ofthe invention.

The shaped glass sheet of the invention has at least one corner, andincludes those shown in FIG. 6.

A glass sheet structure may be constructed with the two or more shapedglass sheets of the invention all having approximately the same radiusof curvature. Such a glass sheet structure is composed of at least twoglass sheets, a spacer which defines a space between the glass sheets,and a means of filling a gap between the glass sheets and the spacer,

each of said sheets being a shaped glass sheet having at least onelinear bend and two straight sections extending from the bend,characterized in that a radius of curvature of an outer circumference ofthe bend transverse to the bend is at most 4 times a thickness of theglass sheet and the straight sections of the shaped glass sheet aresubstantially flat and

the sheets having approximately the same radii of curvature.

Because the shaped glass sheets with a very sharp bend and very flatstraight sections are used in the structure, the structure such as awindow, a show window or a display case does not distort a substancewhich is seen through there. Thus, the structure has excellentproperties which are not expected in the prior art. The aforesaidprocess of the invention makes it possible to produce Just the sameshape of bent glass sheets with high reproductivity. Accordingly, thestructure of glass sheets mentioned above is also produced with highreproductivity as compared with the prior art.

In the glass sheet structure, the glass sheets are each fixed,preferably, in substantially parallel with each other. A space betweenglass sheets depends upon use, but is preferably 3 to 60 mm, morepreferably 3 to 30 mm, particularly 4 to 24 mm. The space between glasssheets means a distance between the straight sections of glass sheetswhich face to each other.

FIG. 7 is a schematic view of a glass sheet structure of the invention.A cross-sectional view is also shown in the lower part of the drawing,which shows a cross section of the glass sheet structure at line I-I' orII-II' in the schematic view. Inner glass sheet, 1, and Outer glasssheet, 2, both have a thickness of 4 mm and are spaced with a space of 6mm between their straight sections.

In the outer glass sheet, a side parallel to the linear bend is 400 mmlong, and sides perpendicular to the bend are both 400 mm long. Theouter glass sheet and the inner glass sheet are spaced with a spacer, 3,which has a width enough to secure a space between the glass sheets andis disposed along the circumference of the glass sheets. The spacer maybe anything as long as it can secure the glass sheets with apredetermined space, but is preferably a hollow square pillar made ofmetal such as aluminium, soft iron and stainless steel or plastics, asshown in the cross-sectional view. Also as shown in the cross-sectionalview, two glass sheets are sealed with a sealing material such as butylrubber, 6, and a calking material such as silicone rubber andpolysulfurized synthetic rubber, 5, via the spacer. Any other means maybe used to fill a gap between the glass sheets and the spacer. Forinstance, use is made of a unit which has a binding member tomechanically fix glass sheets directly to a spacer. It is preferred thatthe inside of the spacer, 3, is completely or partly filled with adrying agent, 4, such as silica gel and molecular sieve. The sealingagent, 6, is to prevent moisture from penetrating into the space betweenthe sealed sheets and, thus, to prevent the glass from clouding. Thedrying agent, 4, also absorbs a trace moisture which penetrated into thesealed space of the glass sheets during assembling. The glass sheetstructure of the invention may be installed by setting its sealedcircumference in a frame such as a window frame which is subsequentlyfixed.

The invention will be further explained with reference to the followingExamples.

EXAMPLE 1

A float method rectangular glass sheet (soda lime glass, softening point740° C.) was provided, which was 800 mm long, 400 mm wide and 4 mmthick. A bend line of 2 mm wide and 20 μm thick was applied on onesurface and both side faces of the glass sheet at the center of thelongitudinal direction in the transverse direction with a silver typeelectrically conductive material, D-1230 (Kai), Fujikura Kasei Co., inscreen printing. Solid terminals were connected to the bend line on theside faces. Then, a rod shape seeds heater of 15 mm in diameter wasplaced 40 mm above the bend line in parallel to the bend line. Also,heat transfer heating units comprising flexible heaters were placed onthe whole surfaces of the glass sheet excerpt areas spaced at most 50 mmfrom the bend line transverse to the bend line. This heat transferheating units clamped the glass sheets on their surfaces to function asa glass sheet-holding mechanism.

First, heat transfer heating was started to heat the whole glass sheetto 450° C. Then, irradiation heating to an area near the bend line wasstarted, followed by applying a current of 50 Hz, AC 38 volts to thebend line for 2 minutes to heat the glass along the bend line to 800° C.The glass sheet reached a temperature profile as shown in FIG. 1. Then,the glass sheet was bent by the glass sheet-holding mechanism in aboutone minute until the two straight sections were at a right angle. Afterthe bending, the shaped glass sheet was annealed.

A radius of curvature of the outer circumference of the bend transverseto the bend in the shaped glass sheet thus produced was 5 mm, i.e., 1.2times the thickness of the glass sheet, in the straight sections. Therewas seen almost no transitional area in which the sheet would slightlywarp and loose flatness. The straight sections of the shaped glass sheetmaintained its original flatness as in the starting float method glasssheet, and exhibited absolutely no increase in optical seeing-throughdistortion and reflection distortion.

EXAMPLE 2

A float method square glass sheet (soda lime glass, softening point 740°C., annealing point 540° C.) of a size of 1,000 mm×1,000 mm and athickness of 5 mm was provided. A bend line with the pattern indicatedin FIG. 3, I, was applied with a constant thickness of 20 μm in screenprinting with a carbon type electrically conductive material (Bany HyteF-525W-1, Nippon Kokuen Shoji Co.) in a center of a surface of the glasssheet. Solid terminals were contacted to the bend line at the end faces.

The glass sheet was then held by a glass sheet-holding mechanism havingarms to which plates were attached to sandwich a glass sheet, and heateduniformly in an electric furnace to 470° C. Subsequently, an alternatingcurrent of 50 Hz was applied to the bend line in a power pattern asindicated in FIG. 8. At a point of time of 60 seconds after turning onelectricity, d, the glass along the bend line was at a temperature of800° C. Then, the heating power was decreased immediately, and the glasssheet-holding mechanism started to bend the glass sheet so that theangle between the straight sections of the glass sheet became 90 degreesin about 60 seconds. FIG. 9 shows a temperature profile along line X₁ X₂transverse the bend line at the point of time of 60 seconds afterturning on electricity. Areas outside points, P₁ and P₂, spaced 15 mmfrom the bend line on the right and left sides were at temperatures nothigher than 550° C. After the bending, the shaped glass sheet wasannealed.

A radius of curvature of the outer circumference of the bend transverseto the bend in the shaped glass sheet thus produced was 6 mm, i.e., 1.2times the thickness of the glass sheet, in the straight sections. Inareas spaced at least 10 mm from the bend line transverse to the bendline, there was seen no transitional area in which the sheet wouldslightly warp and loose flatness. The straight sections of the shapedglass sheet maintained its original flatness as in the starting floatmethod glass sheet, and exhibited absolutely no increase in opticalseeing-through distortion and reflection distortion.

As seen above, the process of the invention using arc discharge heatingmakes it possible to heat a narrow area of a glass sheet along a bendline to at least a softening point in a very short period of time.

EXAMPLE 3

Two bent glass sheets produced as in Example 1 were layered with a spaceof 6 mm between the straight sections via a hollow square pillar spacemade of aluminium. The whole space in the hollow spacer was filled withsilica gel as a drying agent. Gaps between the glass sheets and thespacer were filled with butyl rubber as a sealing material. A spacebetween the glass sheets outside the spacer was filled with siliconerubber as a calking agent. Thus, a glass sheet structure as indicated inFIG. 7 were prepared.

The glass sheet structure had a very sharp bend, and flat straightsections which were not distorted and had a very good seeing-throughproperty. After the use in a long period of time, cloud which would becaused by moisture in the sealed space between the glass sheets was notseen.

What we claim is:
 1. A shaped glass sheet having at least one linearbend and two straight sections extending from the bend, characterized inthat a radius of curvature of an outer circumference of the bendtransverse to the bend is at most 4 times a thickness of the glass sheetand the straight sections of the shaped glass sheet are substantiallyflat, and wherein the thickness of the glass sheet is 3 mm or more. 2.The shaped glass sheet as claimed in claim 1, wherein the radius ofcurvature is at most twice as much as the thickness of the glass sheet.3. The shaped glass sheet as claimed in claim 1, wherein there is atransitional area between the bend and each straight section and a widthof the transitional area is at most twice the thickness of the glasssheet.
 4. The shaped glass sheet as claimed in claim 3, wherein thewidth of the transitional area is at most the thickness of the glasssheet.
 5. The shaped glass sheet as claimed in claim 1, wherein aninterior angle between two adjacent plane parts is between 60 and 160degrees.
 6. A glass sheet structure composed of at least two glasssheets, a spacer which defines a space between the glass sheets, and ameans of filling a gap between the glass sheets and the spacer,each ofsaid sheets being a shaped glass sheet having at least one linear bendand two straight sections extending from the bend, characterized in thata radius of curvature of an outer circumference of the bend transverseto the bend is at most 4 times a thickness of the glass sheet and thestraight sections of the shaped glass sheet are substantially flat andthe sheets having approximately the same radii of curvature.
 7. Theglass sheet structure as claimed in claim 6, wherein the glass sheetsare substantially parallel to each other.
 8. The glass sheet structureas claimed in claim 6, wherein the space between the glass sheets is 3to 60 mm.